1. Field of the Invention
The present invention relates to a method used in a wireless communications system and related communication device, and more particularly, to a method of handling random access procedure with multiple access points and component carriers or with the relay deployment in a wireless communications system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs). The UE of the LTE system can transmit and receive data on only one carrier component at any time.
Toward advanced high-speed wireless communication system, such as transmitting data in a higher peak data rate, LTE-Advanced is standardized by the 3rd Generation Partnership Project (3GPP) as an enhancement of LTE system. LTE-Advanced targets faster switching between power states, improves performance at the cell edge, and includes subjects, such as bandwidth extension, coordinated multipoint transmission/reception (COMP), multiple input multiple output (MIMO), relay deployment, etc.
For bandwidth extension, carrier aggregation is introduced to the LTE-Advanced for extension to wider bandwidth, where two or more component carriers are aggregated, for supporting wider transmission bandwidths e.g. up to 100 MHz and for spectrum aggregation. According to carrier aggregation capability, multiple component carriers are aggregated into overall wider bandwidth, wherein UE can establish multiple links corresponding to the multiple component carriers for simultaneously receiving and/or transmitting on each component carrier.
The COMP is considered for LTE-Advanced as a tool to improve coverage of high data rates, cell edge throughput, and system efficiency, which implies dynamic coordination among multiple geographical separated points. That is, when an UE is in a cell-edge region, the UE is able to receive signal from multiple cells, and the multiple cells can receive transmission of the UE.
In addition, a relay node in the LTE-Advanced system is considered to improve the coverage of high data rates, group mobility, temporary network deployment, the cell-edge throughput and to extend coverage. The relay can be deployed at the cell edge where the eNB may not be unable to provide required radio quality/throughput for the UEs that shall be served by the eNB or at certain location where radio signals of the eNB may not cover.
Moreover, with the relay deployment in the LTE-Advanced system, continuous uplink/downlink transmission/reception opportunities are not feasible due to interference consideration of in band relaying operation (namely eNB-to-relay link operates in the same frequency spectrum as the relay-to-UE link). In order to handle the interference problem, a “gap” concept is introduced so that the eNB or UE during the gap may not expect any relay transmission. For example, the relay-to-UE transmission is performed only during normal subframes, whereas the eNB-to-relay transmission is performed only during multi-media broadcast over a single frequency network (MBSFN) subframes.
However, with consideration of the relay deployment, the COMP, and carrier aggregation in the LTE-Advanced system, several scenarios are described as follows.
In the first scenario, under relay deployment, a UE may be able to hear both the base station(s) and the relay(s), or only hear either base station(s) or the relay(s). In contrary, the network (e.g. base station or relay) has a difficulty in justifying whether the UE can receive signals from the base station or the relay, especially when UE in IDLE mode. Sometimes, only UE knows signal strength by measurement for the base station and the relay. Or, the network determines whether the UE receives signals from the base station or the relay only according to the measurement report from the UE. Consequently, it makes control function, scheduling and coordination more important factors. Judging from the scenarios, for a random access procedure in a communications system with relay deployment, the network currently has no way to justify whether the UE shall access the relay rather than base station or not (and vice versa), thereby the system performance may be downgraded without the well implemented control function, scheduling and coordination of the network.
In the second scenario, the LTE-Advanced system does not define how to applied configurations such as power control parameters for physical random access channel (PRACH) access, random access radio network temporary identifiers (RA-RNTIs), physical down link control channels (PDCCHs), temporary cell radio network temporary identifiers (T-CRNTIs), back-off timers, random access response (RAR) window and contention resolution timers, for the multiple access points or component carriers.
In the third scenario, during a random access procedure, a RAR message of the random access procedure includes response to preambles where each preamble is assigned with different T-CRNTI or UL grant. Meanwhile, contention among preambles used by the UEs could happen due to the same preamble shared among UEs. The network makes use of different T-CRNTIs or resources (for PRACH transmission or UL transmission) for different access points or component carriers, or for distinguishing which access points or component carriers UE accesses to or would like to access. However, a number of T-CRNTIs or resources (e.g. time/frequency, preamble, root sequence) may not enough to differentiate the association of the access points or component carriers. Thus, the UE may not know how to recognize the RAR message including duplicated T-CRNTIs, or resources. That is, the LTE-Advanced system does not specify how the UE shall deal with the situation of insufficient resources.
In the fourth scenario, after UE transmits random access preamble for a random access procedure, the UE shall wait for the RAR message with RA-RNTI potentially addressed to the RAR message according to the accessed PRACH resource until RAR window (e.g. [RA_WINDOW_BEGIN—RA_WINDOW_END]) expires. However, when the UE is under control by a relay, it is possible that due to in band relaying operation, the relay (or even base station) does not transmit RAR to the UE before window expiry (e.g. UE does not expect any downlink transmission during base station to relay transmission) if not well scheduled (e.g. base station may not realize when the relay would like to response to the UE at DL or when the relay request the UE for UL transmission) or due to some purposes (important transmissions from base station to relay node). In addition, the relay may postpone transmitting a contention resolution message due to in band relaying operation. How the UE deal with these situations is never concerned. Here the link between relay and UE is the access link while a link between base station and relay is the backhaul link. While relay and UE involve in the random access procedure performed by the UE, the random access procedure can also be performed by the relay on the backhaul link between base station and relay. In other words, the relay, as also a mobile device, can reuse UE's random access procedure and play a role similar to the UE while performing random access procedure with the base station (e.g. base station to the relay is similar to the relay to the UE or base station to the UE).
Therefore, the fourth scenario between relay and UE can be reconsidered as the scenario between base station and relay. After relay transmits random access preamble for a random access procedure, the relay shall wait for the RAR message with RA-RNTI potentially addressed to the RAR message according to the accessed PRACH resource until RAR window (e.g. [RA_WINDOW_BEGIN—RA_WINDOW_END]) expires. However, when the relay is under control or coverage of a base station, it is possible that due to in band relaying operation, the base station does not transmit RAR to the relay before window expiry (e.g. relay does not expect any downlink transmission from the base station during relay to UE transmission) or the base station transmits RAR in a subframe but the relay doesn't expect any DL transmission in the subframe, if not well scheduled (e.g. base station may realize the UL/DL transmission scheduled by the relay or base station itself) or due to some purposes (e.g. important transmissions from relay to UE). In addition, the base station may postpone transmitting a contention resolution message due to in band relaying operation. How the relay deal with these situations is never concerned.
In the fifth scenario, if the UE receives the RAR from the network, the UE makes use of an uplink (UL) grant 6 ms later after reception for UL transmission of message 3 for a contention based RA procedure. However, due to in band relaying operation, the UE should not transmit message 3 at UL to relay if the relay is transmitting to the base station at the same subframe, or the relay just ignores the UL transmission of the random access procedure from the UE. LTE-Advance system does not define how the relay informs the in-band UL transmission to the UE so that the UL grant should not be applied or should be postponed with a new grant.
Similarly, the fifth scenario between relay and UE can be reconsidered as the scenario between base station and relay. If the relay receives the RAR from the network (e.g. base station), the relay makes use of an uplink (UL) grant 6 ms later after reception for UL transmission of message 3 for a contention based RA procedure. However, due to in band relaying operation, the relay should not transmit message 3 at UL to the base station if the UE is scheduled transmit to the relay at the same subframe, or the base station just ignores the UL transmission of the random access procedure from the relay. LTE-Advance system does not define how the base station informs the in-band UL transmission to the relay or how the relay realizes there will be a scheduled UL transmission to base station before the relay schedules the UE for a UL transmission at the same subframe, so that the UL grant should not be applied or should be postponed with a new grant or as a new grant.